This invention relates generally to an ion cyclotron resonance (ICR) cell, and more particularly to an ICR cell with large ion storage capacity.
Ion cyclotron resonance is well known and has been employed in numerous spectroscopy devices and studies. Generally, these devices store the ions to be analyzed in cells of various configurations which are disposed in a uniform magnetic field. Gaseous ions in the presence of the uniform magnetic field are constrained to move in circular orbits in a plane perpendicular to the field (cyclotron oscillations). The ions are not constrained in their motion parallel to the field. As a consequence, various cell configurations have been adopted to retain the ions within the cell. For example, the cell may include end plates which have dc voltages applied thereto, or it may be of an open cell design such as described by Beu et. al., xe2x80x9cOpen trapped ion cell geometries for FT/ICR/MS, Int. J. Mass Spectrom. Ion Processes, 112 (1992), 215-230. Another cell configuration is described in U.S. Pat. No. 5,019,706.
The frequency of the circular motion is directly dependent upon the charge-to-mass ratio of the ions and the strength of the magnetic field. When orbiting ions trapped within the cell are subjected to an oscillating electric field, disposed at right angles to the magnetic field, the ions having a cyclotron frequency equal to the frequency of the oscillating electric field are accelerated to increasingly larger orbital radii and higher kinetic energy. Because only the resonant ions absorb energy from the oscillating field, they are distinguished from the non-resonant ions upon which the oscillating electric field has a substantially negligible effect. The oscillating ions are detected by separate electrodes which have image current induced therein by the oscillating ions. In another example, the cell does not include separate detection electrodes, and is operated in a switched mode. A two-electrode ion trap is described by Marto, et al., xe2x80x9cA Two-Electrode Ion Trap for Fourier Transform Ion Cyclotron Resonance Mass Spectrometryxe2x80x9d, Int. J. Mass Spectrom. Ion Processes, 137 (1994), 9-30.
Generally, the ions are excited by a pulsed wave form having multiple frequencies whereby ions of different masses undergo ion cyclotron resonance. Comisarow and Marshall in U.S. Pat. No. 3,937,955 describes the operation of an ICR cell excited with waveforms having multiple frequencies in what is known as a Fourier transform mode (FT-ICR). It has been recently demonstrated that one of the primary limitations to obtaining accurate mass measurement for FT-ICR is space charge-induced shifts of the cyclotron frequency. These shifts can be minimized by having a reproducible number of ions during each scan (Winger, et al., xe2x80x9cHigh Throughput, High Speed, Automated Accurate Mass LC-FT/MS Analysisxe2x80x9d, Proc. 46th ASMS (1998), p. 516).
Other FT-ICR systems are less sensitive to space charge-induced shifts and therefore produce more reliable mass accuracy data. For example, the infinity cell (Caravatti et al., xe2x80x9cThe Infinity Cell: a new Trapped-ion Cell With Radio-frequency Covered Trapping Electrodes for Fourier Transform Ion Cyclotron Resonance Mass Spectrometryxe2x80x9d, Org. Mass Spectrom., 26 (1991), 514-518) (Allemann et al., xe2x80x9cIon Cyclotron Resonance Spectrometerxe2x80x9d, U.S. Pat. No. 5,019,706), which uses a linearized dipolar field which allows a greater ion excitation radius and the use of xe2x80x9cside-kickxe2x80x9d injection (Caravatti, Pablo, xe2x80x9cMethod and apparatus for the accumulation of ions in a trap of an ion cyclotron resonance spectrometer, by transferring the kinetic energy of the motion parallel to the magnetic field into direction perpendicular to the magnetic fieldxe2x80x9d, U.S. Pat. No. 4,924,089), which gives the ions an initial non-zero magnetron radius. Both of these features contribute to lower ion density and thus a reduced sensitivity to space charge-induced frequency shifts.
The primary drawback to a non-zero initial magnetron radius is that the acquired signal will contain significant harmonic content and other modulations of the fundamental signal (Chen et al., xe2x80x9cAn off-center cubic ion trap for Fourier transform ion cyclotron resonance mass spectrometryxe2x80x9d, Int. J. Mass Spectrom. Ion Processes, 133 (1994), 29-38). One method which allows the formation of an off-axis ion cloud without the observation of higher-order harmonics is the use of a two-electrode trap such as described by Marto et. al., xe2x80x9cA Two-Electrode Ion Trap for Fourier Transform Ion Cyclotron Resonance Mass Spectrometryxe2x80x9d, Int. J. Mass Spectrom. Ion Processes, 137 (1994), 9-30. This trap has been shown to be an order of magnitude less sensitive to space charge shifts than a standard cubic trap. The primary disadvantage of the two-electrode trap is the severe axial ejection caused by the parametric excitation and significant axial fields.
It is an object of the present invention to provide an improved ICR cell.
It is a further object of the present invention to provide an ICR cell in which the ion cloud is off-axis.
It is a further object of the present invention to provide an ICR cell in which space charge-induced shifts are minimized.
The foregoing and other objects of the invention are achieved by an ICR cell which comprises two concentric elongated electrodes and trapping electrodes disposed at the ends of the concentric electrodes to form an ion trapping volume in the space between the concentric electrodes.